Conventionally, the development of a high efficiency, high resolution and small scale direct conversion type compound semiconductor for use in a radiation detector element is progressing. Among these semiconductors, attention in recent years is paid to cadmium telluride (CdTe), cadmium selenium telluride (CdSeTe), cadmium zinc telluride (CdZnTe), and cadmium zinc selenium telluride (CdZnSeTe), which are II-VI compound semiconductors as powerful materials. These are referred to below as CdTe-based semiconductors. Compared to semiconductors such as silicon (Si) or germanium (Ge), these semiconductors have the advantage of (1) a high detection efficiency of radiation due to a relatively large atomic number, and (2) a cooling device is not necessary since the effect of a thermal noise current is small because the band gap is large. When radiation incidents while applying an electric field to a semiconductor attached with electrodes, electrons and holes are generated in the semiconductor and proceed in mutually opposite directions along the applied electric field. In this way, radiation is detected by extracting the induced current as a signal.
Conventionally, vacuum deposition or plating and the like are known as methods of forming a metal electrode for connecting a circuit to the surface of a CdTe substrate or CdZnTe substrate. Recently, electroless plating is often used to form a gold electrode or a platinum electrode by immersing a CdZnTe substrate in a plating solution containing chloroauric acid or chloroplatinic acid, and depositing gold or platinum or alloys thereof to certain sections of the substrate surface (see Japanese Laid Open Patent Publication No. 2003-142673, Japanese Laid Open Patent Publication No. 2001-177141, Japanese Laid Open Patent Publication No. H08-125203, Japanese Laid Open Patent Publication No. H07-038132, Japanese Laid Open Patent Publication No. H03-248578 and Japanese Laid Open Patent Publication No. H03-201487).
Pt is suitably used in the case of forming a radiation detector using this type of CdTe-based semiconductor. Furthermore, although a solder is used depending on the process when a signal line for extracting a signal from an electrode is bonded, so as not to degrade characteristics by permeating the solder through the Pt layer as far as the CdTe-based semiconductor part and does not, a means is adopted for controlling the permeation of the solder by forming a thick Ni layer above a Pt electrode and forming a thin Au layer above the Ni layer (Under Bump Metal: UBM). A means such as plating or sputtering is used in the formation of a Pt layer and UBM structure.
However, when forming an UBM structure above a Pt layer, peeling occurs when forming the UBM structure in the case where adhesion between a CdTe-based semiconductor and the Pt layer is insufficient, and even when formed, peeling of the electrode sometimes occurs during the solder jointing process. In addition, although direct bonding of a signal line to a Pt layer using a metal with a low melting point such as In or by Au wire bonding is performed without using the UBM structure, peeling at the time of bonding the signal line sometimes occurs since adhesion between a CdTe-based semiconductor and Pt layer is weak.